> Basically I want to add some truss/bracing action but with minimal > parts/size.

I guess I would try something like 5mm stainless braided cable attached end to end on the underside. The trick will be establishing the correct tension. You could try using a heavy bass string and a bass tuning gear could be mounted at the head stock. But that may well not be strong enough to counterbalance the topside strings.

You might just have to grit your teeth and peel up the wires on the backside so you can mount a metal plate on the surface of the underside.